Direction finding antenna system



1953 .1. R. FORD ET AL DIRECTION FINDING ANTENNA .SYSTEM 2 Shets-Sheet.1

Filed Oct. 2, 1950 22 EEC.

EEC.

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N E 6 z m 6 INVENTO JokrzzRFord fmorzfiollzrzgerg' z I ATTORNEY Wit560M542 Aug. 11,.19'53 Filed Oct. -2, 1950 J. R. FORD ET AL DIRECTIONFINDING ANTENNA SYSTEM 2 Sheets-Sheet 2 aldmz! BY g f ATTORNEY PatentedAug. 11, 1953 UNITE DERECTION FINDING ANTENNA SYSTEM ware ApplicationOctober 2, 1950, Serial No. 187,992

3 Claims.

This invention relates to radio echo detection systems and moreparticularly to such systems employing a wave guide feed.

A type of antenna which is now well known is that utilizing a waveguideto which is coupled a plurality of antenna elements. These antennaelements may comprise spaced slots in a hollow pipe waveguide wallthrough which energy within the waveguide is radiated. Again, aplurality of dipoles or the like may be suitably coupled at spacedintervals along a waveguide. The antenna slots may overlaplongitudinally, thus to form a continuous staggered slot. In the past,such systems have been excited so that all of the elements radiatesimultaneously during at least a major portion of the transmission orreception.

It is an object of the present invention to provide a new and novelmethod of operation of such an antenna.

It is another object of the invention to provide a novel combination ofan antenna structure employing a plurality of such antenna elements.

A further object of the invention is to provide a novel radio echodetection system in which such an antenna is employed.

In accordance with the invention, the operation of the antenna isentirely different from the operation which has been employedheretofore. The novel operation is secured by associating with theantenna a pulsed energy generator, the pulses of energy in their travelthrough the waveguide being short in length with respect to the totalwaveguide length. Thus, an entirely novel system of operation is securedin which only a fractional portion or" the antenna elements areradiating at any moment of time and at no tim are all of the elementsthereof simultaneously excited.

The foregoing objects, advantages and novel features of the inventionwill be more apparent from the following description in which likereference numerals refer to like parts and in which:

Fig. 1 is a perspective view of a slotted waveguide antenna;

Fig. 2 is another perspective view of a slotted waveguide antenna withan associated circuit schematically indicated;

Fig. 3 is a cross-sectional view of a somewhat different type ofwaveguide antenna which may be utilized in practicing the invention;

Fig. 4 is a schematic view of another system embodying the invention;

Fig. 5 is a sketch illustrative of the geometry of the systemillustrated in Fig. l;

Fig. 6 is illustrative in block diagram form of a circuit which may beused in connection with the system of Fig. 4.

Fig. '7 is a perspective view of an assembly which may be used in anovel radio echo detection system embodying the invention; and

Fig. 8 is illustrative of a variation in the system of Fig. 4.

Referring now more particularly to Fig. 1, a long rectangular waveguideIii has transverse slots I2 cut in one of the broad walls M. These slotsl2 are spaced longitudinally a half guide wavelength, the slots beingsuccessively staggered on each side of the median line of the broad wallid, as shown, thus adjacent slots have radiation reversed in phase dueto the spacing of half guide wavelengths, but also reversed due to thestaggering current on opposite sides of the broad wall center line beingin opposite directions. Thus adjacent slots radiate in-phase. Wave-guideID has an absorptive matched termination it at one end thereof. Theother end is supplied with electromagnetic energy from a transmitter 18having a very high frequency of operation and being modulated in veryshort pulses. The energy is supplied to wave-guide ii] from thetransmitter I8 to input end l9 through a T-R arrangement 20 of any ofthe various .well-known kinds whereby during transmission energy issupplied to waveguide ill but upon the cessation of transmission theenergy returning from waveguide H1 is switched to a receiver 22. Let theperiod of one of the pulses from transmitter l8 be represented by T. Theenergy will travel through waveguide It at a velocity which may beassumed to equal substantially the group velocity 1). Let the length ofthe waveguide H! from the input end to the termination l6 be L. Then theenergy advances from the input end toward the termination l6 and thepulses are so short that the length of the pulse, in the waveguide IB issubstantially qual 11 t. and is less than L, and preferably only a smallfraction, say about 6 or less of L. Thus the length L is substantiallylonger than the distance traversed in the waveguide It by the energy inthe tim of a duration single pulse. Other antenna elements than theslots shown might be used. Accordingly, only a portion of the antennaelements spaced along the guide lengths are radiating energy at a time.As an example, suppose that there are a hundred apertures l2. The lengthof the pulse may be only ten times the spacing of the aperture, andconsequently at th most ten adjacent apertures l2 are emitting energy atany instant of time. After the first ten nearest the input end !9 of thewaveguide are radiating energy, a few moments in time later the firstapertureis not radiating energy because the pulse a has passed it andthe next ten are radiating energy. This continues until the last ten areradiating energy and thereafter as the energy strikes the matchedabsorbing termination H5 it is absorbed so that successively the lastnine apertures, the lasteightapertures, etc..,.near. the end 96, areradiating. The: effect is best seen by refering to Fig. 2. The radiationpattern from Waveguide it may assume a shape such as that illustrated bythe lobe 24 which at a later time may be in the position illustrated bythe; dotted line lobe 25. The waveguide. l orv apertures. or

both may be graduated in sizeinorder to assure;

that each of th apertures lzradiateisubstane tially the same amount ofpoyer as every other aperture. Waveguide I0 may be of a length to extendfor example, from one en'd 'of an' air-- plane runway to the other. Theenergy isitransmitted from transmitter I 8 and the lapse in time beforeit is-returned to: receiver 22 will give information regarding the timerequiredforthe pulse to leave the transmitter; pass through thewaveguide into the p'arti'cular'position substantially directlyunderneath anairplane, thence-to reach. the airplane, and for the echoto return vertically to waveguide l0 and thenceto the TR arrangement 20'to receiver 22;

An alternative waveguide arrangement is'shown in Fig. 3 in which awaveguide 30' semi-circular in cross-section feeds dipoles 32spaced"equally along the length of the. Waveguide 30. These are arranged toradiate in phase. The waveguide 30 may be substituted for the waveguideHi bearing in mind again that it is to be long with respect to thelength of the pulse being transmitted through. it, appropriate spacing,of the antenna elements 32, and appropriate orientation of the guide todirect. the energy from the dipoles 32 in the vertical plane (or. suchother plane a may be desired) Referring now more particularlyto Fig. 4',three waveguides 40, 42., and Mlarespacedequally and laid out inparallel lines. parallel to therunway of an-airplane landing field.These. three Wave.- guides each may be similar. to waveguide It havingradiated elementswhichin. the plane longitudinal thereto atthefrequency,of .operation are quite narrow and are restricted to an. area directlyabove theslots momentarily ratiating. In the transverse plane, the.waveguidesfl, 42, and M may have respectively radiation patternssuch asit, 48, and 553. The transmitter l3, whichmay be the same asthetransmitter ofFig. l, feeds one end'of the waveguideAlthroughaT.-R.arrange.- ment 52.

Areceiver 56 is: also. connectedtothe T-A ar.-. rangement toreceivetheecho-signal as from an airplane 61 from waveguideA-Z and. for.simplicity receivers '3and 62' are shown connectedto waveguides ii? andit respectively to receive. the signals therefrom.However,,waveguides.40 and 44 could also be connectedito the input. ofreceiver 68 instead of beingcorr-ected assshown.v Receivers 53, Eli, and52- may be connectedto anoscililoscope 6d (the face only of. whichisshown in the drawing) as a common element todisplay the signalsreceived therein. It will be understood that the oscilloscope sweepcircuitifi5-may be syn:- chronized with the transmitter:by-connection 66so that the sweep startsfrom thedeft with transmission of a pulse ofenergyby' transmitter 18 and moves uniformly with: time to the right. Apip it will appear at'the initiation of the sweep-representing themainbang, and three; other pips,

one 12 and two pips 'lzappear-at somezdistancei to the right (assumingthe sweep to progress from left to right) representing the return ofsignals from waveguide 42 and waveguides 16, and 4 respectively. Thevelocity of the signal in the waveguides, each of which may be presumedto be constructed similarly to the others, may be known or measured. Itwill be assumed that an approaching plane "M is at an altitude smallcompared to the slant range from the receiver ends, that is the ends ofwaveguides it, 42, and M to which the receivers 53', (iii, and 52 areconnected. In order to. center the plane ti on the runway down. whichthe waveguides are laid, it is now only necessary to. direct the pilotto bring his plane toward one or the other side of the field until twoof the pips F2 (from guides 48 and M) merge into one. The distancebteween the pips it 311(11171'18 pip 12' on the oscilloscope screen 5 isproportional to the ground range of the aircraft plus its altitude.

The system of Fig. 4 is operative only when the plane is over thelanding field, that is above the assembly of waveguides 691,52 and 3%.For ground control approach (GCA) systems, the range indicated by thedistance between pips if! and i2 approximates the true range of theaircraft, and approaches this true range more closely as the aircraftapproaches touchdown over the waveguideAZ. Moreover, in mose cases, theerror involved issmall. However, if desired, the altitude of the planemay be measured by employing a slight modification. of the system,whereby the error may be (in effect) subtracted from the range toindicate the true range.

Referring to Fi 5, the applicable geometry is illustrated. When the pipsi2 merge, the distance. between the pip l2 and the merged pips l2isindicative of a time of travel which in turn indicatesa distanceq.

Now

q=b-c (l) and Therefore:

/2 (a /q-q) (3) Therefore, the altitude C can be found by standard,computer techniques. For example, a special potentiometer may be woundleaving the requisite variations with q, at least over alimitedrange,.to. give C, whcnmotion of a pick-up arm is linear withq.6n the oscilloscope St, Fig. 4, for example, may be impressed a voltageproportionalto C derived from such a computer. Fig. 6 i1- lustrates howthe voltage proportional to q may be derived. From receiver till asignal is supplied via a connection at to a flip flop circuit 32 whichis flipped in one-direction by the signal indicated byv pip 12;" andinthe other direction by merged pips-l2; fed by connection 853 also. Therectangular wave out-put of nip-flop circuit 82 is fed to a triangularshaper S4 (or integrator). The average output voltageof the triangularshaper 84 is proportional'to the distance (1.

The system of Fig. 4 functions preferably when the plane 61 is somewhereover the runway, as pointed out hereinbef-ore. Standard known methodsmay be used to bring the plane in thus far, or for example, the spacingof slots 52 only at the far end of the runway, may be altered to directthe energy radiated from the end remote from the receiver to be directedlongitudinally away from the waveguide. A continuous slot near the endof eachof waveguides 4t, 42, and 44, such as illustrated in Fig. 8, mayalsobe. usedfor this;

purpose. Assuming, waveguides 40, 42, and 44 to be so constructed,causing a plane to move so that pips 72 coalesce lines up the course ofthe plane with the runway.

Referring now more particularly to Fig. 7, there is illustrated a hollowpipe waveguide I which is preferably substantially rectangular incrosssection to avoid Inoding difficulties. A series of antenna elementsI02 spaced around the waveguide I00 of Fig. may be excited by energy inthe waveguide. Waveguide I60 is preferably bent into a substantiallycircular shape and the antenna elements i022 are placed on the side ofthe circular guide facing away from the interior of the circle. Thewaveguide I00 may be laid substantially the ground plane and with theelements I02 thus facing outwardly the energy travels radially from thecircle in which the waveguide I00 is shaped. A waveguide with acrosssection such as that of Fig. 3 may here be employed to advantagewith antenna elements at a slight angle to the ground plane. Atransmitter I8 is connected to a T-Rarrangement I04 which in turn isconnected to the end I38 of waveguide 00. The receiver H0 is connectedthrough the T-R arrangement N14 to the end I08 of waveguide I00. Theother end H2 of waveguide N50 is connected also to a receiver III). Anindicator, which may be cathode ray oscilloscope I I4 shown in face viewin the drawing, has the signals from the receiver displayed thereon. Thesweep is from left to right, linear with time and initiated to startwith each pulse transmission from transmitter it. A connection I I6 isemployed to synchronize the sweep. The receiver pulses are applied tothe vertical plates.

In operation, the transmitter I8 feeds a pulse into the end I08 ofwaveguide I00. This pulse again is so short in time duration that at anymoment of time only a fractional portion of the elements I02 areradiating. Accordingly the pulse travels out in a circular sweepstarting at the end I03, travelling circumferentially around waveguideI00. The system is intended to give warning indications of approachingplanes at a considerable distance from the waveguide I00. Accordingly,the time of travel to/2 of the signal path through waveguide I00 may beconsidered short and negligible compared with the time of travel throughspace to the object to be indicated and the return of the echotherefrom. Accordingly the distance from the main bang I22 of the echopips I24 and I26 on screen of oscilloscope H0 is representative of thedistance from the waveguide I00 to the plane. The angular location ofthe object to be indicated may be determined from the differentialbetween the pips I24 and I26. More accurately, let tr be the time oftravel of energy radiated from a point I I8 on waveguide I 00 nearestsome reflecting object such as an airplane. Let is be twice the time oftravel of energy in the waveguide I00 from the point IIB to the nearestend H2. Then the elapsed time between initiation of transmission of apulse (main bang) and the first echo is:

Similarly, the elapsed time between the main bang and receipt of thesecond echo is:

1'2: /gta-f-tr-I-Vz (tin-to) =tT+ /zto where to is twice time of travelof the pulse from one end I08 to the other end II2 of the energy in thewaveguide I00. Thus ta (proportional to the azimuth angle) isproportional'to the differ ence in sweep distance between the two echopips plus a constant, and tr (range) is proportional to the difierencein sweep distance between the main bang and the second pip minus aconstant. Computer circuits are well known which give a solution ofthese equations. Again, as a simple means, a specially woundpotentiometer may be employed. To derive sensing, it may be desirable toshape the output pulses of the different receivers differently toresolve the ambiguity of Whether the first pulse is received from theleft or right semi-circle of the waveguide I00 as viewed in the drawing.

Referring now more particularly to Fig. 8, there is illustrated amodification of the construction of the end of waveguide I0 01 Fig. 1remote from connection to the receiver. For a short distance from thisend, say a, length equal to the number of simultaneously radiatingelements, the waveguide is opened at the top by a continuous slot I20.From slot I20 the energy leaves substantially without reflection.However, the slot acts in this instance as an end fire array, causingthe energy to be projected forwardly, instead of vertically, at an angleinclined from the horizontal to a degree determined largely by the phasevelocity in this portion, that is, the portion having slot I20. Thus ina system as that of Fig. 4, there may be detected a warning signal byreturned reflections of planes approaching the runway from a positionwell in advance of the waveguide I0. If this variation is used, oneshould preferably have some means for distinguishing high altitudes froma distance in advance of the end of the waveguide I 0 as perhaps byanother search radar. More simply, only one of the waveguides, say 42 ofFig. 4 is so made. Then the absence of an echo from the other waveguidesis an indication that signal is being received from the slot portion ofthe one waveguide 42, and that the reflecting object or plane is anadvance of the runway.

It will be apparent that there is disclosed a new and novel method ofoperation of a waveguide antenna, as well as new and novel combinationsutilizing such a waveguide.

What we claim as our invention is:

1. In a system for radiating or receiving energy and having a long leakywaveguide, the method of operation comprising the step of exciting saidwaveguide with a succession of pulses of high frequency energy the timeduration of each of which is substantially less than the time requiredfor said pulse energy to traverse the length of said waveguide.

2. In a system for radiating energy, said system having a long waveguideand a substantial plurality of antenna elements equally spaced along thelength of said waveguide and coupled thereto, the method of operationcomprising the steps of exciting said waveguide with successive pulsesof high frequency energy the time duration of each of which issubstantially less than the time required for said pulse energy totraverse the length of said waveguide, and exciting each of saidelements successively along the waveguide length with said pulses ofenergy but substantially less than all of said elements at any one time,thereby to establish an antenna pattern for said radiation which travelsin space in the direction of motion of said energy through thewaveguides.

3. In a system for receiving energy, said system having a long waveguideand a substantial plurality of antenna... elements equally; spaced;along thelength; of: saidi waveguide. and; coupled thereto; the: methodof. receivingjpulse energy: comprising the steps of energizing;successively andprogressively a. few ofthe adjacent. antennaelementsspaxced along said waveguide with.each' of aasuccession. ofpulses of. high. frequency energy the; duration. ofeach of- Which isubstantiallyless than. the :time required for'said. pulse energy totraverse the length-ofsaid.Waveguide, and energizing said waveguide withsaid successive pulses of=- energyfrom. the: energized. an-- tennasthereby to couple: energy inthewaveguide moving in. the direction inwhich the elements.

are-successively energized.

JOHN. R. FORD. WALDON PEARSON BOLLINGER.

References Cited in the file of this patent UNITED STATES PATENTS

